In the cellular communication art, land mobile radio networks transmit and receive high frequency signals (greater than 800 MHz) via antennas located at land mobile radio sites. In order to maximize the geographic area for coverage of the signal, the effective radiated power (ERP) must be maximized. The ERP is the product of the power input to the antenna times the gain factor of the antenna; that is, the solid angle direction of the transmission and reception path of the antenna.
It is known in the art that in order to have high ERP while reducing the absolute power into the antenna, the antenna must necessarily have a high gain factor. In order to increase the gain of an antenna, the physical aperture, that is the height and width of the antenna, must increase and the antenna's beam, as defined by the solid cone angle, must necessarily occupy fewer steradians. Thus for instance, an antenna might have a vertical beam width of 4.degree., while the horizontal beam width may be 30.degree.. These beam widths define the antenna's radiating beam solid cone angle. Typically, the smaller the beam solid cone angle, the higher the gain of the antenna.
For cellular communication applications, it is generally required, depending upon the location of the land mobile radio site, to cover 360.degree. of azimuth while the vertical beam width may only be 4.degree. degrees in order to effectively cover a geographic area. However, in order to cover 360.degree. of azimuth and maintain high gain, it is typically necessary to use twelve antennas with 30.degree. of horizontal beam width each. The cost of such antennas and the availability of mounting space for such antennas present significant difficulties. Furthermore, this number of antennas can present windloading problems at the antenna tower, as well as provide a detrimental visual appearance.
In order to overcome the problems associated with providing twelve antennas with 30.degree. of horizontal beam width each, it is known in the land mobile radio industry to produce multiple antenna patterns (a multi-beam pattern) out of a common aperture using a Butler-matrix feed network. Such a matrix consists of a phasing network with N inputs and N outputs, where N can be any integer greater than one. This phasing network serves to take each of the N inputs and divide the signal amongst the N output ports with each output port having a fixed phase offset with respect to the other output ports. By properly adjusting the phases between adjacent antennas, the output lobe from the antenna can be electrically steered to the left or right in a controlled fashion. Each of the N inputs creates a different set of phase shifts on the N outputs and therefore results in N distinct "beams" from a common aperture. Such an antenna is sometimes referred to as a phased array antenna. FIG. 1 illustrates an example of this phase-shifting arrangement for 8 inputs and 8 outputs (N=8). A discussion of the Butler-matrix feed is presented in "Antenna Engineering Handbook", 2nd edition, Richard C. Johnsen and Henry Jassick, McGraw-Hill Book Company, pps. 20-56 through 20-60.
Since it is not necessary to have separate antenna apertures to make all of the required antenna beams, the Butler-matrix feed approach greatly reduces the problems associated with the visual appearance of a plurality of antennas, with the concomitant reduction in windloading, as well as some cost savings with regard to mounting space. One approach for an antenna driven by such a Butler-matrix is shown in FIG. 2, which illustrates four (4) rows or sets of four co-linear arrays of radiating elements, yielding a 4.times.4 panel of radiating elements.
The beam widths, sidelobe levels and grating lobes of an antenna comprising N co-linear arrays of N radiating elements driven by an N beam Butler-matrix feed network are defined by the physics of the overall antenna system. Thus the spacing between the co-linear arrays of radiating elements (in wavelengths of the radiating or received energy) drive the grating lobes while the sidelobes are driven by the spacial Fourier transform of the antenna aperture width and the radiating element spacing within each of the co-linear arrays. For four vertically polarized co-linear arrays of radiating elements at 0.5 wave length horizontal spacing (between adjacent arrays), the sidelobes are approximately 7 dB below the main lobe. Even if the number of co-linear elements per array is increased vertically, such as to 8, such an arrangement does not change the sidelobe level relative to the main lobe. A -7 dB sidelobe is a significant problem for cellular communications due to the fact that it does not provide the azimuthal beam pattern required for land mobile radio system operation.
It has been shown through the use of Monte Carlo Analysis Programs conducted at U.S. West New Vector Group in Bellevue, Wash. that -10 dB sidelobe levels are the maximum levels which can be adequately tolerated for such land mobile radio system operation. Thus, the standard arrangement of an antenna with four co-linear arrays of four radiating elements each, connected to a Butler-matrix feed network is not suitable for such communication.
One approach to dealing with the problems associated with high sidelobe levels is by controlling the power delivered to each co-linear array of radiating elements, and reducing the power level to the outermost beams while maintaining a high power level for the inner beams to thereby decrease sidelobe levels. FIG. 3 is an example of such an antenna system utilizing an amplifier between each output of the Butler-matrix which feed the individual co-linear arrays of radiating elements. The gain of the amplifiers is selected such that lower power is provided to the outermost co-linear arrays of radiating elements, e.g., -3 dB with respect to the central co-linear arrays. A reduction in power to the outermost co-linear arrays relative to the inner co-linear arrays is sometimes referred to as power tapering. A problem associated with such an arrangement is that the amplifiers are active elements and are susceptible to failure. Such an arrangement also presents a significant maintenance problem if the amplifiers are located in a tower of a mobile radio base site adjacent to the antenna. It is much simpler to service and maintain an amplifier that is located remotely from an antenna, for example at the base of a tower in the cellular base site. Another problem associated with such an arrangement is the potential distortion (intermodulation distortion) introduced by the multitude of amplifiers. Additionally, such an arrangement is more expensive and complicated.
A second method of decreasing the sidelobe levels of a phased array antenna is disclosed in commonly owned U.S. Pat. No. 5,589,843. This patent discloses decreasing the sidelobe level by reducing the number of co-linear radiating elements at the outer edge of the multi-co-linear array antenna which is driven by a microstrip implemented Butler-matrix network feed. As illustrated in FIG. 4, in such an antenna, the absolute gain of the antenna decreases slightly because the physical aperture is slightly smaller. The reduction of the number of co-linear elements for the co-linear arrays toward and at the edges of the antenna is sometimes referred to as space tapering. Such space tapering is highly desirable with regard to the reduction of sidelobe levels.
Such a space tapered antenna provides the significant advantage of reduced sidelobe level at the expense of providing a beam pattern which is not as uniform as the beam pattern produced by such an antenna having an equal number of radiating elements in the various co-linear arrays, such as the antenna of FIG. 2.